However, the structure in which the magnetic field is adjusted in strength to adjust the wavelength of the radiation may have following several problems.
As illustrated in (A) of FIG. 1, in case of the planar undulator, the magnetic field may be easily adjusted by using the permanent magnet or electromagnet. When the permanent magnet is used, the array of the permanent magnets may be physically spared or narrowed in the vertical direction to change the size of the gap space of the undulator. In case of the planar undulator, even though the permanent magnet is used, the gap space of the undulator may easily change in size. However, in case of the helical undulator, unlike the planar undulator, it may be structurally difficult to adjust the gap between the magnets (see (B) in FIG. 1).
When the electromagnet is used, the mechanical movement may not be necessary at all, and thus, only the current may be adjusted as described above. However, since the magnetic field of the electromagnet is a relatively weak when compared to that of the permanent magnet (thus, since large current has to be applied to form a strong magnetic field that is similar to that of the permanent magnet, it is impossible to form the strong magnetic field at room temperature by using a wire having a small diameter), the radiation may not be effectively generated as well known. In case of the helical undulator, since the gap is hard to be adjusted in size to adjust the strength of the magnetic field, thereby adjusting the wavelength of the radiation as described above, the electromagnet has to be used. Thus, it may be difficult to obtain the radiation having a desired high output power with a compact size because of the small-sized wire.
In addition, in the method for adjusting the wavelength of the radiation by adjusting the strength of the magnetic field, the output power of the radiation as well as the wavelength of the radiation may change.
Due to these several problems, studies about the undulator structure in which the magnetic field change in period, but does not change in strength, to adjust the wavelength of the radiation have been continuously carried out. FIG. 2 illustrates undulator technologies in which a magnetic field is adjusted in period according to the related art.
An undulator including a mechanical link device for adjusting a distance between magnets is disclosed in U.S. Pat. No. 6,858,998 (“Variable-period undulators for synchrotron radiation”, 2005 Feb. 22, hereinafter, referred to as a prior art 1). In the prior art 1, the distance between the magnets may change, i.e., the magnetic field may be adjusted in period to adjust a wavelength of radiation. Thus, since it is possible to use a permanent magnet, the above-described problems when the electromagnet is used may be solved. (A) of FIG. 2 is a schematic view of the link device according to the prior art 1. As illustrated in (A) of FIG. 2, an angle between links of the link device may be adjusted to adjust the distance between the magnets. However, in case of the prior art 1, since it is very difficult to realize fine movement due to the structural characteristics thereof, it may be very difficult to precisely adjust the distance between the magnets. Thus, it may be difficult to finely adjust the wavelength of the radiation. In addition, the prior art 1 relates to a planar undulator. This structure may not be applied to the helical undulator as it is and also be very difficult in design change to be applied to the helical undulator.
A variable-period structure in new viewpoints for the planar undulator is disclosed in the paper “Variable-Period Permanent Magnet Undulators” (Vinokurov, N. A. et al., 2011 Physical Review Special Topics-Accelerators and Beams 14(4), art. no040701, hereinafter, referred to as a prior art 2). (B) of FIG. 2 illustrates a structure and principle of the undulator disclosed in the prior art 2. In the prior art 2, permanent magnets and ferromagnetic materials each of which has a size less than that of each of the permanent magnets are alternately disposed. Here, the permanent magnets may be magnetized parallel to a progress direction of an array of the permanent magnets. Here, the permanent magnets may be alternately magnetized in the progress direction of the array of the permanent magnets so that the magnetized directions of the upper and lower arrays are symmetrical to each other.
When disposed as described above, the ferromagnetic materials may generate strong magnetic lines in an upward or downward direction by concentrating the magnetic fields generated from the adjacent permanent magnets. Here, the magnetic line formed on the ferromagnetic material may have a shape to allow a central line between two permanent magnets to form a symmetrical central line as illustrated in the enlarged view of (B) in FIG. 2. Here, in the prior art 2, as illustrated in the enlarged view of (A) in FIG. 2, the ferromagnetic materials are separated from each other with respect to the symmetrical central line. As a result, a repulsive force may be generated between the two separated ferromagnetic materials. Thus, a compressive force may be physically applied only in one direction from the outside to easily adjust the period of the magnetic field due to the repulsive force acting between the ferromagnetic materials.
However, it may also be impossible to apply the structure of the prior art 2 to the helical undulator. In case of the helical undulator, the pair of planar undulators has to be vertically disposed to cross each other as illustrated in (B) of FIG. 2, like the planar undulator illustrated in (b) of FIG. 2. Here, in cross-sections in an axis direction of the helical undulator, the arrangements of [the ferromagnetic materials in a vertical direction/the permanent magnets in a horizontal direction] or [the permanent magnets in a vertical direction/the ferromagnetic materials in a horizontal direction] are realized on the same plane. Thus, if the ferromagnetic materials are cut at a central position between the permanent magnets, the permanent magnet disposed in a direction perpendicular to the ferromagnetic material on the same plane has to be cut. However, if the permanent magnet is cut, the permanent magnet may be formed only as two permanent magnets to generate an attractive force therebetween. That is, since the attractive force is applied between the separated permanent magnets even though the repulsive force is applied between the separated ferromagnetic materials, it may be impossible to effectively spread the distance between the magnets. As described above, the structure of the prior art 2 may be optimized for the planar undulator, but may not be applied to the helical undulator.